Many common downhole applications rely on detection of thermal or epithermal neutrons. One of the applications is neutron porosity, which is part of what is known as “triple combo” and is a standard for any logging tool string. Downhole tools therefore often contain a neutron source and several thermal and epithermal neutron detectors.
The strengths of sources used to create neutrons are limited due cost and safety concerns (for example from material activation). In addition, chemical sources are limited in size by government regulations; whereas, the availability of electronic neutron sources, particularly in oilfield applications, are limited by reliability and thermal management. To compensate for limited neutron source strength, a common requirement for neutron detectors for oilfield applications (e.g., downhole) is high efficiency. As space within an oilfield measurement tool, or sonde, is restricted, a detector package is also limited in size (depending on application, approx. 13-76 mm diameter and 13-200 mm long), which makes the efficiency requirement more difficult to meet.
Another complication in oilfield applications is that neutron measurement tools are constantly moving. In such applications, signals have to be recorded promptly without any delays from internal processes or data acquisition. For certain types of measurements employing pulsed neutron sources, the detectors have to be particularly fast. An example of such a measurement is “Sigma” in which the neutron signal decay is measured on a time scale of tens of microseconds with a resolution as good as one microsecond. Therefore, an additional requirement for such detectors is a reasonably short time decay, which is in the microsecond range. Furthermore, the detectors should withstand the rugged borehole environment which means shock, vibration, elevated pressures and a range of temperatures from about −40° C. to about 200° C. The number of requirements, such as those mentioned above, has traditionally left only a small number of choices available for neutron detection.